But the supplement has no effect if it is started more than 8 weeks into the pregnancy.
In the study, women who took folic acid supplements from four weeks before conception to eight weeks into pregnancy had a 40 per cent lower risk of giving birth to children with childhood autism (classic autism).
"It appears that the crucial time interval is from four weeks before conception to eight weeks into pregnancy," states Pal Suren, MD and doctoral fellow at the Norwegian Institute of Public Health.
The study is based on the Norwegian Mother and Child Cohort Study (MoBa) and the Norway Autism Birth Cohort Study (ABC). It covered a total of 85 176 children born in the period 2002-2008.
The results of the study of the correlation between intake of folic acid supplements and childhood autism indicate that the lower risk is only associated with this specific supplement and not with the consumption of food or other supplements.
The researchers have also found a correlation between folic acid supplements and the reduced risk of severe language delay by the age of three. Such language problems are common in connection with autism but may also occur with many other conditions.
"It will be a tremendous breakthrough if it turns out that folic acid also prevents other developmental disorders," Dr Suren believes.
The study was recently published in the Journal of the American Medical Association (JAMA).